We seek funding to purchase a Bruker ESP-300E X-band electron paramagnetic resonance (EPR) spectrometer to replace a 17 year old Varian E-109 instrument that can no longer be maintained. It is proposed to equip this instrument with an additional Q-band bridge and cavity, Q-and X-band Li He cryostats, and a dual mode cavity. The instrument is a key element in the work funded by 13 NIH grants from the "major" and "other" users groups. The major users group consists of Profs. John D. Lipscomb (P.I.), Lawrence Que, Jr., Marian T. Stankovich, and William B. Tolman. These individuals and many of the "other" users are members of the Center for Metals in Biocatalysis at the University of Minnesota. Many collaborations within this group have led to joint publications and student training. The EPR instrument plays a major role in the viability of this Center and the collaborations it engenders. Many different types of problems centered around metals in biology are represented in the research of major users group. These include investigations of both mononuclear and dinuclear iron cluster containing enzymes and relevant model systems; investigation of nitrogen metabolism in Cu and heme containing enzymes and model systems; and investigations of metallo- flavoprotein systems. All of these studies require stable temperatures near that of liquid helium. The Q band accessory is requested due to the need to investigate heme, nonheme, Mn, and Cu systems at more than one frequency in order to interpret the complex spectra that arise from coupling or small zero field splitting. Several of the enzymes and models under investigation exhibit novel EPR signals from integer spin systems. These signals have proven to be important tools in the investigation of key intermediates in the catalytic cycles of enzymes such as methane monooxygenase (MMO). Integer spin signal intensity is enhanced about 3-fold when observed using the requested dual mode cavity due to a difference in the selection rules versus those of the half integer spin systems that are the usual targets of EPR studies. Also, the background signals due to half integer spin systems are eliminated when the dual mode capacity is used. Integer spin signals often have intensity at zero magnetic field because the X-band quantum is comparable in energy to the zero field splitting. Use of the higher energy Q-band quantum will, in principle, allow more complete investigation of these integer spin systems. The current EPR system has been heavily used for its entire service lifetime and continues to be in high demand. It has been made available to the entire research community free of charge. The new instrument will allow us to continue this tradition and facilitate expansion into new areas such as ENDOR that cannot be readily implemented with the current instrument. A new laboratory designed specifically for EPR is part of the medical school basic sciences building currently under construction. This will allow the instrument to be placed adjacent to 5 other molecular structure core facilities as well as the relevant technical personnel.